The Bush administration has scaled back plans to quickly build a “virtual fence” along the U.S.-Mexico border, delaying completion of the first phase of the project by at least three years and shifting away from a network of tower-mounted sensors and surveillance gear, federal officials said yesterday.

Technical problems discovered in a 28-mile pilot project south of Tucson prompted the change in plans, Department of Homeland Security officials and congressional auditors told a House subcommittee.

Though the department took over that initial stretch Friday from Boeing, authorities confirmed that Project 28, the initial deployment of the Secure Border Initiative network, did not work as planned or meet the needs of the U.S. Border Patrol.

The announcement marked a major setback for what President Bush in May 2006 called “the most technologically advanced border security initiative in American history.” The virtual fence was to be a key component of his proposed overhaul of U.S. immigration policies, which died last year in the Senate.

Investigators for the Government Accountability Office had earlier warned that the effort was beset by both expected and unplanned difficulties. But yesterday, they disclosed new troubles that will require a redesign and said the first phase will not be completed until near the end of the next president’s first term.

Those problems included Boeing’s use of inappropriate commercial software, designed for use by police dispatchers, to integrate data related to illicit border-crossings. Boeing has already been paid $20.6 million for the pilot project, and in December, the DHS gave the firm another $65 million to replace the software with military-style, battle management software.

In an interview, Gregory L. Giddens, the department’s executive director for the border effort, confirmed that “we . . . have delayed our deployment as we work through the issues on Project 28. While there is clear urgency of the mission, we also want to make sure we do this right.”

Boeing has said that the initial effort, while flawed, still has helped Homeland Security apprehend 2,000 illegal immigrants since September. It estimated in 2006 that it would spend $7.6 billion through 2011 to secure the entire 2,000-mile southern border, an ambition that was meant to win support from conservatives for legislation creating a guest-worker program and a path to legalization for 12 million illegal immigrants.

But officials said yesterday that they now expect to complete the first phase of the virtual fence’s deployment — roughly 100 miles near Tucson and Yuma, Ariz., and El Paso, Tex. — by the end of 2011, instead of by the end of 2008. That target falls outside Boeing’s initial contract, which will end in September 2009 but can be extended.

The virtual fence was to complement a physical fence that the administration now says will include 370 miles of pedestrian fencing and 300 miles of vehicle barriers to be completed by the end of this year. The GAO said this portion of the project may also be delayed and that its total cost cannot be determined. The president’s 2009 budget does not propose funds to add fencing beyond the 700 or so miles meant to be completed this year.

A crackdown on drug and human smuggling rings at the U.S. border with Mexico has sparked an increase in violence against border patrol agents.

U.S. Department of Homeland Security officials say the number of assaults is up this fiscal year after more than doubling between 2004 and 2007, USA Today reported Wednesday.

“We’ve had occasions of people shooting at agents, trying to run agents down with vehicles, throwing large rocks or pieces of brick or concrete at agents, which can actually be fatal,” says Homeland Security Secretary Michael Chertoff.

Chertoff says the escalation is the result of stepped-up enforcement that aims to put smugglers out of business.

Most of the assaults have involved “rockings” in which rocks, bricks and objects are thrown at agents.

Agent Shawn Moran of the Border Patrol in Imperial Beach, Calif., told USA Today the rockings are “like biblical stoning. This is like what they used to do to kill people.”